On a basic level, yes, both Josh and git filter-branch do essentially the same thing. The difference being that Josh is much faster not just compared to git filter-branch but also compared to all the other similar tools out there, especially when run repeatedly in the same repo.

Also being a server it does not require any installation or resources on the developers machine.

In addition to that over time more features where added that git-filter branch does not have, most notably "josh workspaces" which is a DSL for repo transformations.